This invention relates, in general, to digital logic interface circuits and, more particularly, to interface circuits from other types of logic families into integrated injection logic (I.sup.2 L) circuitry, and methods of interfacing with I.sup.2 L circuitry.
Integrated injection logic (I.sup.2 L) offers many advantages over other types of logic for certain applications. However, other logic types have features which make them more desirable in other applications and often the two logic families will have to interact with one another. Since most logic families, including the common types such as T.sup.2 L, and MOS logic, are not directly compatible with I.sup.2 L circuitry, it is necessary to provide some sort of interface circuit.
In the past, the interface circuit has consisted of a series of transistor stages which voltage shift the output from a given circuit type to make it compatible with the I.sup.2 L input requirements. Such circuitry is illustrated by C. M. Hart et al, in "Bipolar LSI Takes a New Direction With Integrated Injection Logic", Electronics, Oct. 3, 1974, at page 111. Specifically, FIG. 7 of Hart et al's article shows a four transistor interface circuit from a T.sup.2 L to an I.sup.2 L device. However, the interface circuits used in the past have had several drawbacks. For example, the several stages of transistors results in signal delays from the driving circuitry into the I.sup.2 L circuitry. Moreover, these devices require a relatively large area on the semiconductor substrate. Also, each logic type driving the I.sup.2 L such as T.sup.2 L or CMOS or NMOS requires a separate type of interface circuit. Thus, in the past it has been generally impractical to build a universal interface circuit on a semiconductor substrate for accepting whichever type of interface a user would choose as an input to the I.sup.2 L circuitry.
Therefore, it can be appreciated that a digital logic to I.sup.2 L interface circuit which is simple, fast, and applicable to several different types of logic is highly desirable.